Ribonuclease inhibitor

Leucine Rich Repeat
Top view of porcine ribonuclease inhibitor, showing its horseshoe shape.[1] The outer layer is composed of α-helices and the inner layer of parallel β-strands. The inner and outer diameters are roughly 2.1 nm and 6.7 nm, respectively.
Identifiers
Symbol	LRR_1
Pfam	PF00560
Pfam clan	CL0022
InterPro	IPR003590
SMART	SM00368
SCOP2	1bnh / SCOPe / SUPFAM
Available protein structures: Pfam   structures / ECOD   PDB RCSB PDB; PDBe; PDBj PDBsum structure summary PDB 1a4y​, 1dfj​, 1z7x​, 2bex​, 2bnh​, 2q4g​

Ribonuclease inhibitor (RI) is a large (~450 residues, ~49 kDa), acidic (pI ~4.7), leucine-rich repeat protein that forms extremely tight complexes with certain ribonucleases. It is a major cellular protein, comprising ~0.1% of all cellular protein by weight, and appears to play an important role in regulating the lifetime of RNA.^[2]

RI has a surprisingly high cysteine content (~6.5%, cf. 1.7% in typical proteins) and is sensitive to oxidation. RI is also rich in leucine (21.5%, compared to 9% in typical proteins) and commensurately lower in other hydrophobic residues, esp. valine, isoleucine, methionine, tyrosine, and phenylalanine.